Coatings are often applied to medical appliances, such as pacemakers, vascular grafts, stents, and heart valves, to have desired effects and increase their effectiveness. These coatings may deliver a therapeutic agent or drug to the lumen that reduces smooth muscle tissue proliferation or restenosis. Furthermore, medical devices may be coated to provide beneficial surface properties, achieving enhanced biocompatibility and to improve surface properties such as lubriciousness. Balloon delivery systems, stent grafts and expandable stents are specific examples of medical appliances or implants that may be coated and inserted within the body. Stents such as described in U.S. Pat. No. 4,733,665, are tiny, expandable mesh tubes supporting the inner walls of a lumen used to restore adequate blood flow to the heart and other organs.
Conventionally, coatings are applied to the stent in a number of ways including, though not limited to, dip coating, spin coating or spray coating. Spray coating processes generally require an apparatus to securely hold and rotate the flexible, tiny stent structure during the coating operation to allow a reproducible and homogeneous coating application on the whole surface.
However, holding devices known from the prior art have several drawbacks which may result in low volume production of medical devices, damage to the fragile stent structure, inhomogeneous coatings, uncoated areas, coating accumulations, and the like. Coating accumulations, such as shown in FIG. 13, can lead to severe damages of the coating due to a possible loss of the coating during loading, transportation, and/or deployment of the stent. Coating defects, such as uncoated areas and coating thickness variations on the stent surface may compromise the implant's effectiveness due to potential complications arising from an inhomogeneous distribution of the therapeutic agent at the target site.
Stent holding devices, as described in U.S. Pat. No. 6,605,154, comprising a mandrel passing all the way through the inner hollow body of the stent to support the stent via support members, which partially penetrate into the opposing sides of the hollow body of the stent by incrementally moving at least one support member closer to the other, can have several disadvantages.
When using such stent holding devices there may be a risk of coating defects at the ends of the stent due to the design of the support elements. The clamping force can vary from stent to stent, which may lead to sagging or buckling of the stent. Mandrels having a small diameter and a comparatively long length of approximately 40-80 mm may easily bend resulting in a runout of the stent. In most cases the run out of the mandrel is several mm which may effect the coating weight consistency. Moreover stent holding devices, as described in U.S. Pat. No. 6,572,644, comprising members projecting out of a body to contact the stent may not center and secure the stent sufficiently.
Runout, sagging and buckling of the stent may cause an inhomogeneous coating thickness, coating defects on the stent surface and coating weight deviations. Coating consistency may vary from stent to stent depending on runout and positioning accuracy of the support members.
In addition, coating defects including uncoated areas may arise when stent holding devices are used having a structure which interferes with the spray plume as described in WO Pat. No. 2004/008995.
Damage of the coating may also occur after completion of the coating process during handling and inspection. Inspection of medical devices generally requires dismounting the stent from the holding device being used during the coating process in order to mount the stent to an inspection fixture that typically contacts the outer surface of the stent.
Finally, stent holding devices known by the prior art are not designed to support and/or coat multiple stents simultaneously or to be used for subsequent inspection of the coated stents.